The present invention relates to a photopolymerizable composition, and more particularly to a photopolymerizable composition improved in the strength of cured film obtained therefrom.
In the manufacture of printed circuit boards, dry film type photoresist is superior to liquid type resist in that the former can easily be formed into a ohotoresist layer having a uniform film thickness, it can be applied to the so-called tenting process as a through-hole etching resist, etc. According to the tenting process, holes called "through-hole" are previously perforated, a metallic layer of copper or the like is formed in the through-hole, a dry film is laminated on both sides of a two-sided substrate having conductive layers on its both sides, it is exposed to light pattern-wise so as to form a resist film a little greater than the through-hole diameter on the necessary circuit pattern and through-holes, it is developed, and thereafter it is etched to remove the unnecessary metal layer. At the time of etching, the metal inside the through-holes is protected by the resist film provided on the through-holes, so that both sides of the printed circuit board thus formed are brought into an electrical continuity. Although the dry film resist used for this purpose is required to be tightly adhesive to substrate stably under the conditions of development and etching and to be prevented from penetration of etching solution into through-holes, no satisfactory dry film resist has hitherto been obtained from the viewpoint of general performances. Although an increased quantity of light exposure at the time of pattern formation generally brings about an improvement in tent film strength, an increase in the quantity of light exposure is accompanied by a decrease in the degree of resolving power which makes it difficult to form a high density pattern and further it prolongs the time period necessary for exposure which decreases the productivity.
Although methods for improving the adhesion to substrate in the land parts around through-holes and thereby enhancing reliability were proposed in Japanese Patent Application Kokai (Laid-Open) Nos. 59-113,432, 60-12,543, 59-166,944 and 60-19,135, these methods comprise adding a substance accelerating adhesion into the photosensitive layer, which often causes various failures in the succeeding process such as failure of development, tailing of resist, and the like.
With the aim of improving the strength of cured photosensitive film after exposure, addition of halogen type additive was proposed in Japanese Patent Application Kokai (Laid-Open) Nos. 60-15,972, 57-60 327 and 59-78,339, and the method is useful for improving sensitivity and color-developing performance, too. However, this method is disadvantageous in that it causes a decrease in storage stability in photoresists having a relatively high sensitivity. Particularly when the photosensitive layer is contacted with copper surface, this method tends to promote the dark reaction in unexposed areas.